Justin, from the Qu’Appelle area, phoned me yesterday and asked how he should be using his big heater to dry his very wet wheat. Here is his situation: He is renting a big industrial diesel heater, 1.4 million btu/hr. and 5,000 CFM. It burns 10 gallons of diesel every hour. He is going to connect the heater to 4 bins, each holding 4500 bushels of wheat @ 20% MC. How long will it take to heat his grain by 1 C ?
We will consider only one bin of 4500 bushels x 60 lbs/bu divide by 2.204 lbs/kg to get 122,505 kg of wheat. The specific heat of wheat is about 1.36 kJ/kg C . To raise the temp of the wheat by 1 C, we need 1.36 kJ x 122,505 kg = 166,606 kJ.
Our heater puts out 1,400,000 btu/hr. but we only get 1/4 of this, 350,000 btu/hr and there is 1.055 kJ/btu or 369,250 kJ/hr. put into one bin and since we need 166,606 kJ to raise the temp of the grain by 1 deg C; it will take 166,606/369,250 = 0.45 hours or 27 mins to raise the temp by one degree. Let’s call it a half hour or 30 mins. I told Justin that he should not heat the grain by more than 6 C to prevent problems with condensation. He would be able to do this in 3 hours.
Justin also wanted to know how hot the air would be coming out of the heater; or the temperature rise. We have to know the specific heat of air, 1kJ/kg and the weight of the air which is 0.0807 lbs per cubic foot. The heater has an air flow of 5000 CFM, so in one hour, 300,000 cubic feet of air will pass through it which weighs 300,000 x 0.0807 = 24210 lbs or 10,984 kg. To raise the temp of this air by 1 C requires 10,984 kJ/hr but we have 369,250 kJ/hr. So 369,250 / 10,984 = 33 º C. If the air entering the heater was 0 ºC, it would come out around 30 ºC That’s not too bad, you won’t be scorching the wheat.